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for Nuclear Waste Immobilization
I. Bardez, Daniel Caurant, J.-L. Dussossoy, P. Loiseau, C. Gervais, F. Ribot,
D.R. Neuville, N. Baffier, C. Fillet
To cite this version:
I. Bardez, Daniel Caurant, J.-L. Dussossoy, P. Loiseau, C. Gervais, et al.. Structural Characterizations
of Rare Earth-Rich Glasses for Nuclear Waste Immobilization. ATALANTE 2004 (advances for future
nuclear fuel cycles), Jun 2004, Nîmes, France. �hal-00177963�
Structural Characterizations of Rare Earth-Rich Glasses for Nuclear Waste Immobilization
I. Bardez1,2,*, D. Caurant2, J.L. Dussossoy1, P. Loiseau2, C. Gervais3, F. Ribot3, D. R. Neuville4, N. Baffier2,
C. Fillet1
1 Commissariat à l'Energie Atomique, Centre d'Etudes de la vallée du Rhône, DIEC/SCDV/LEBM,
30207 Bagnols-sur-Cèze, France
2 Laboratoire de Chimie Appliquée de l'Etat Solide (UMR 7574), ENSCP, 11 rue Pierre et Marie Curie,
75231 Paris Cedex 05, France
3 Laboratoire de Chimie de la Matière Condensée (UMR 7574), Université Pierre et Marie Curie,
4 place Jussieu, F-75252 Paris Cedex 05, France
4 Laboratoire de Physique des Minéraux et des Magmas (UMR 7047-CNRS-IPGP), Université Pierre et
Marie Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France Email: isabelle-bardez@enscp.jussieu.fr
Abstract – New nuclear glass compositions, able to immobilize highly active liquid wastes arising from
high burn-up UO2 fuel reprocessing, are being studied. Investigations are being performed on rare
earth-rich glasses, known as durable matrices. After a preliminary study, a basic glass composition was selected (Glass A, wt. %) : 51.0 SiO2 – 8.5 B2O3 – 12.2 Na2O – 4.3 Al2O3 – 4.8 CaO – 3.2 ZrO2 – 16.0 Nd2O3.
The aim of this study is to determine the local environment of the rare earth in this glass and its evolution according to neodymium. To achieve this objective, glasses were prepared from the baseline Glass A with variable neodymium oxide amounts (from 0 to 30 wt. % Nd2O3). By coupling characterization methods
such as EXAFS (Extended X-Ray Absorption Fine Structure) spectroscopy at the neodymium LIII-edge,
optical absorption spectroscopy, 11B, 27Al MAS-NMR and Raman spectroscopy, pieces of information on the rare earth surroundings in the glass were obtained.
INTRODUCTION
New nuclear fuels with high discharge burn-up (60 000 MWj/t) have been studied for some years in France. Reprocessing of this nuclear spent fuel will generate HLW more concentrated in fission products and minor actinides than nowadays. Therefore, new glass compositions have to be developed, able to immobilise these wastes as a whole. These matrices must exhibit excellent chemical durability and higher glass transformation temperature (Tg) than those of the
current borosilicate nuclear glasses. As the actinides and lanthanides quantity is expected to be higher in the new spent fuel, our research turned towards rare earth-rich glassy matrices. Nevertheless, lanthanide
aluminoborosilicate glasses (LaBS) or lanthanide aluminosilicate glasses were not chosen in this work because of their high melting temperature (Tm ≥ 1450°C) [1,2]. Indeed, process problems
could arise from the volatility of some fission products during melting and from the limited resistance of stainless steel containers during melt casting at such high temperature. After a preliminary research, an alternative glass composition was selected for this study (Glass A, wt. %) : 51.0 SiO2 – 8.5 B2O3 – 12.2 Na2O – 4.3
Al2O3 – 4.8 CaO – 3.2 ZrO2 – 16.0 Nd2O3. In
this simplified system, more than 75 wt. % of the new HLW are simulated (the other 25 wt. % did not play a fundamental role in the properties of the glass and were thus eliminated for this structural study). Relatively large quantities of sodium and boron oxides were introduced (known as fluxing agents) so as to decrease the melting temperature below 1300°C. In the following work, both environment around the rare earth in Glass A and its evolution according to Nd concentration were studied. To this end, techniques such as EXAFS spectroscopy at the LIII-edge of neodymium, optical absorption
spectroscopy, 11B, 27Al MAS-NMR and Raman
spectroscopy were used.
EXPERIMENTAL
From Glass A, nine samples were prepared, containing 0, 1.3, 2.5, 5, 10, 16 (= Glass A), 20, 25 and 30 wt. % Nd2O3. Two glasses were also
prepared with lanthanum (16 and 30 wt. % La2O3) for MAS-NMR studies. Glasses were
produced after thorough mixing of reagent grade oxide (SiO2, H3BO3, Al2O3, ZrO2, Nd2O3) and
carbonate (Na2CO3, CaCO3) powders. All
glasses were melted at Tm = 1300°C for 3 h in a
with a Tm of 1400°C because of crystallisation
problems).
EXAFS analyses were performed at the LURE-DCI synchrotron source (Orsay, France), using the Nd LIII edge (6.2 keV). All X-ray absorption
spectra were recorded in transmission mode. Data were collected under vacuum at 77 K (liquid nitrogen). EXAFS spectra were extracted with the software developed by A. Michalowicz (EXAFS_98. ppc) [3-4] and data analysis was performed with FEFF-Fit [5], following standard methods based on scattering theory. Simulations of the first and second nearest neighbors were realized by using the theoretical backscattering phase and amplitude functions calculated with the ab-initio code FEFF 8.0 [5]. Nd2O3, Nd2Si2O7
and Ca2Nd8(SiO4)6O2 crystalline phases were
used as model compounds. Due to the large static disorder occurring around neodymium in our glasses, a cumulant analysis [6-7] of the first shell (Nd-O) was preferred. In this case, ρ(R), the radial probability distribution of atoms in the shell, is expressed as a power series of cumulants Ci (1):
∑
∫
∞= ∞ − = 0 0 2 2 ) ( 2 ! ) 2 ( ) ( ln n n n kR i k R C n k i dR e R e R λ ρ (1)Third and fourth cumulants (C3 and C4) are thus
necessary to characterise the large asymmetric distribution of distance. The third cumulant describes the asymmetry of the distribution whereas the fourth cumulant depicts symmetry deviation from Gaussian shape [6-7].
Optical transmission experiments were carried out at low temperature (T ≈ 10 K) on polished glass samples (thickness from 0.5 to 1 mm). Raman measurements were carried out on a T64000 Jobin-Yvon confocal microRaman spectrometer equipped with a CCD detector. The 514.532 nm line of a Coherent 70 Ar+ laser was used for sample excitation. This line operated between 2 W and 0.5 W.
MAS NMR spectra were recorded on a Bruker Avance 400 spectrometer at a frequency of 128.28 MHz using a Doty probe with no probe background for 11B and on a Bruker DRX600
liquid spectrometer (14.1 T) operating at 156.38 MHz and equipped with a Bruker high speed MAS probe-head for 27Al. Chemical shifts were
either determined relative to liquid BF3OEt2 (δ =
0 ppm) or to an acidic aqueous solution of Al(NO3)3 (1M). The spectra were simulated with
the DMFIT program [8] which has been developed specifically for solid state NMR experiments, including simulation of quadrupolar shapes [9].
RESULTS AND DISCUSSION
EXAFS at the LIII-Nd Edge Spectroscopy
Five glasses were analyzed by EXAFS : Nd2.5, Nd5, Nd10, Nd16 (= Glass A) and Nd30. Fourier transforms of the EXAFS signals (Fig. 1) display a steady decrease of intensity with increasing neodymium amount in glass. According to the fits results (Table I), neodymium always appears surrounded by 8 ± 1 oxygen atoms at a mean distance of 2.46 ± 0.03 Å. Moreover,
approximately 3.5 silicon or sodium take place around neodymium ions at about 3.98 ± 0.03 Å. Nevertheless, although boron is not
distinguishable by EXAFS, it is very probable, according to the Bray's model [10] for the SiO2 –
B2O3 – Na2O system, to find boron in the
vicinity of neodymium in our glasses.
Fig. 1. Fourier transforms of Nd LIII-edge
EXAFS spectra of glasses containing 2.5, 5, 10, 16 and 30 wt. % Nd2O3 (T=77 K).
TABLE I. Quantitative results of the EXAFS analysis of the Ndx glasses with Ni, the number of
neighbours, σi, the Debye-Waller factor and Ri,
the distance (Nd-neighbours).
Sample number Nd2.5 Nd5 Nd10 Nd16 Nd30 N 8.1 8.5 7.7 8.0 7.8 R (Å) 2.44 2.45 2.47 2.46 2.48 Νd−Ο σ2 (Å2) 0.020 0.022 0.020 0.021 0.021 C3 (Å3) *10-4 12 14 16 13 17 C4 (Å4) *10-5 41 40 28 29 29 N 3.5 3.5 3.5 3.5 3.5 R (Å) 3.98 3.98 3.99 3.98 3.99 Nd-Si/Na σ2 (Å2) 0.011 0.013 0.014 0.013 0.014
Global quality of fit of 99.7 %
Otherwise, fit tests performed for all glasses with neodymium ions as second neighbours display aberrant fit parameters values (Debye-Waller factor σ > 0.30 Å and quality of fit < 65 %). That means that no phenomena of Nd-Nd clustering at less than 4 Å were detected in these glasses by
0 1 2 3 4 5 0 1 2 3 4 5 6 Nd2.5 Nd5 Nd10 Nd16 Nd30 FT M a g ni tude R(Å) 1st neighbour contribution 2nd neighbour contribution
EXAFS, even for high neodymium contents. In our glasses, only a significant increase of the static disorder in glass is observed with increasing neodymium concentration (represented by σ2 (Debye-Waller factor), C
3 and
C4 cumulants), responsible for the decrease of
intensity observed on the Fourier transforms.
Optical Absorption Spectroscopy
Optical transmission measurements were performed at T ≈ 10 K. At this temperature, only the first of the five Stark levels of the 4I
9/2 ground
state of neodymium is populated. Among all the electronic transitions of Nd3+ ion, two ones are of
particular importance for the present
investigation. Firstly, the transition between the
4I
9/2 ground state and the 2P1/2 excited state
(around 23 225 cm-1, Fig. 2 (a)) brings
information on the variety of sites occupied by Nd3+ ions in the glass and on the degree of
covalency of the bonding (Nd-O). Secondly, the transition between the 4I
9/2 state and the 2G7/2, 4G
5/2 states (around 17 300 cm-1, Fig. 2 (b)),
called hypersensitive, owns shapes strongly affected by changes in the environment around Nd3+ ion.
In our glasses, peaks profiles (inhomogeneous half-width and position in energy) of the 4I
9/2 → 2P
1/2 transition of Nd3+ (Fig. 2 (a)) do not display
any evolution with neodymium concentration in glass. Peaks maxima are all centred at 23 226 cm-1 and bands shapes were fitted with one
Gaussian getting a constant half-width of 120 cm-1. According to these results, it can be put
forward that the (Nd-O) bonding and also the nature of the Nd-surroundings must not show strong evolutions with neodymium content. However, the hypersensitive 4I
9/2 → 2G7/2, 4G5/2
transition (h.s.t.) shape (centred at 17 120 cm-1
and 17 460 cm-1) presents slight evolutions with
neodymium concentration (Fig. 2 (b)). Indeed, shoulders present at the lowest energy side, strongly emphasized at low neodymium concentrations, become progressively less apparent as [Nd2O3] increases, simultaneously
occurring with a band broadening. This can either be due to an increase of the Nd-sites distribution and also an increase of the static disorder as neodymium content increases in glass (also shown by EXAFS), or due to an increase of the mean field strength Z/a2 (Z is the cation
valence and a the sum of the cation and oxygen radius, field strength is composed by Na+, Ca2+
and Nd3+ contributions). Indeed, according to
Dymnikov et al [11], with increasing Z/a2, the
h.s.t. become less resolved and shoulders disappear (for both silicate and borate glasses).
Fig. 2. 4I
9/2 → 2P1/2 (a) and 4I9/2 → 2G7/2, 4G5/2 (b)
transitions of Nd3+ for Ndx glasses at T ≈ 10 K
(1.3, 5, 10, 16 and 30 wt. % Nd2O3).
* indicates shoulders on the band. (The intensities of the peak maxima were
normalised to the same arbitrary value).
11B and 27Al MAS-NMR
MAS-NMR studies were realized on glasses prepared with lanthanum instead of neodymium. Indeed, using Nd3+ (paramagnetic ion) involves
broadened NMR signal shapes due to the dipolar interaction between its electronic spin and the nuclear spin of the studied nuclei.
11B MAS-NMR
Earlier 11B MAS-NMR studies realized on
borosilicate minerals and glasses [12-15] have shown that tetrahedral BO4 and trigonal BO3
sites exhibit distinct chemical ranges. By integration of the area of these two signals, relative intensities of the two types of sites can be determined (Table II). In these glasses, the BO3 units proportion, at a fixed BO4 one,
strongly increases as neodymium concentration increases (Fig. 3).
Fig. 3. 11B MAS-NMR signals of glasses
containing 0, 16 and 30 wt. % of RE2O3.
Intensities were normalised to the same arbitrary intensity of the BO4 shape.
0.32 0.36 0.4 0.44 0.48 0.52 0.56 0.6 0.64 2.29 104 2.3 104 2.31 104 2.32 104 2.33 104 2.34 104 2.35 104 A b s o rb a n c e ( rel . un it s ) wave number (cm-1) Nd30 Nd16 Nd10 Nd5 Nd1,3 (a) 0 0.5 1 1.5 2 2.5 3 1.64 1041.66 1041.68 1041.7 1041.72 1041.74 1041.76 1041.78 1041.8 104 A b s o rb anc e (re l. un it s ) wave number (cm-1 ) Nd1.3 Nd5 Nd10 Nd16 Nd30 (b)
*
*
-20 -10 0 10 20 30 ppm Nd0 (La-Nd)16 (La-Nd)30TABLE II. Proportion of BO3 and BO4 units in
glasses containing 0, 16 and 30 wt. % of RE2O3
(RE = Rare Earth)
Sample number % BO3 units % BO4 units
RE 0 42 58
RE 16 62 38
RE 30 80 20
27Al MAS-NMR
27Al MAS-NMR study allows to determine the
different coordination states of the Al3+ ions in
glasses (Al(IV) around 60 ppm, Al(V) between 25 and 40 ppm or Al(VI) between -15 and 20 ppm). In Ndx glasses, chemical shifts and band shapes do not display any evolution with neodymium content in the glass. At a constant chemical shift of ~ 57 ppm, aluminium ions seem only to be present in four coordination number (Al(IV) units) in all glasses. In oxide glasses, aluminium ions thus form [AlO4]
-entities [16-18].
Raman Spectroscopy
Raman spectroscopy of oxide glasses is interesting especially in the high-frequency envelope [800 – 1250 cm-1], in which T-O
stretch frequencies (with T = Si, Al) occur. As increasing neodymium content in glass, a shift of the SiO4 Qn units to the lower frequencies is
observed (Fig. 4). According to previous investigations [19-20], this shift could probably be due to the creation of Q3(Nd) and Q2(Nd)
units, arising at lower frequencies than the corresponding Q3(Na) and Q2(Na).
Fig. 4. Raman spectra of Ndx glasses.
CONCLUSIONS
In all Ndx glasses, neodymium appears
surrounded by 8 oxygen atoms and ~ 3.5 second neighbors Si / Na / B, with no significant evolutions in the Nd-neighbouring. Only an increase in the Nd-sites distribution is observed
along with neodymium content. No Nd-clustering occurs at less than 4 Å. According to all these results, it appears that neodymium probably imposes its environment to the glass.
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